I'll be mighty impressed if you can get $400 for a used open-mold CX frame. Not saying it's not worth it, just that people are really skeptical about used carbon and really skeptical about open-mold carbon. Put the two together and I think you'll be lucky to get $200.
Regardless of how one feels about open mold carbon, you really do have to go into it with the mindset that the frame is "disposable" and not some kind of long term investment. So since you're likely to take a big $ hit changing, I'd just ride it until it breaks. You'll probably find it's not nearly as fragile as you think it is (it's supposed to be able to handle CX racing after all). It will probably be lots of fun to ride and if you get a year or two out of it then it isn't a total loss.